Magnesium alloys have the lowest density of common engineering metals and therefore they are becoming more and more attractive for various automotive applications. The use of magnesium alloys for power train applications not only significantly reduces the overall vehicle weight, but it also contributes to desired rebalancing of the weight distribution by reducing the weight at the front of the car. This results in improving vehicle dynamics and in creating a commercially attractive product. The manufacturers, therefore, strive to introduce magnesium alloys into power train components, such as gearbox housing, oil pan, transfer case, crankcase, oil pump housing, transmission stator, intake manifold, and others. In addition there are some power train components, e.g. engine cradle and control arm, that require, in addition to good creep behavior, also good properties associated with energy absorption, such as impact strength and fracture toughness, and further also good ductility. A new, cost effective, magnesium alloy with such properties would resolve several critical issues that limit the large-scale application of magnesium castings in the automotive industry. Existing creep resistant alloys, used in high-pressure die casting, are not suitable for large and heavy components, such as engine blocks, which should rather be produced by gravity casting (sand or permanent mold), or by low-pressure casting (sand or permanent mold). Furthermore, there are several power train components, like engine cradle, lower control arm, etc., requiring materials having not only good creep resistance, but also improved energy-absorption properties and ductility.
The strategy of developing gravity casting alloys differs significantly from that for high-pressure die casting alloys. The major mechanisms underlying the properties of high-pressure die casting alloys, comprise strengthening solid solutions due to specific alloying elements, and strengthening grain boundaries due to the rapid cooling under solidification. The stable intermetallics, precipitating during the solidification process, have a eutectic nature and are relatively coarse. On the other hand, the major mechanisms that affect properties of creep resistant gravity casting alloys comprise hardening during the precipitation, and grain boundary strengthening. Thus, when developing creep resistant magnesium alloys for gravity casting, several principles should be taken into consideration. The solid solubility in magnesium of the main alloying elements should be good, and should sharply decrease as the temperature decreases down to ambient temperature. This will enable a marked response to aging. Solubility limits for binary magnesium alloys can be found in “Phase Diagrams of Binary Magnesium Alloy” (eds. A. A. Nayeb-Hashemi and J. B. Klark, Metals Park, Ohio, 1988). Solute atoms should have a low diffusion coefficient in the matrix, to provide strong interatomic bonds and to form the solid solution, which has no response to aging under the working conditions. Good properties at elevated temperatures require thermal stability of the intermetallic compounds, which should have good coherency with the matrix, thus strengthening grain boundaries and effectively forming obstacles against the deformation. The melting point of the precipitate is a good indication of its thermal stability. The first precipitates to nucleate are very often metastable and coherent with the matrix, providing excellent precipitation hardening. As aging progresses, metastable precipitates are transformed into stable equilibrium phases. The morphology of the precipitates is the major factor which affects both ambient strength and creep resistance.
Heat treatment is a very important factor for achieving a required combination of service properties, and should be employed. Solid solution treatment should be performed at the highest practicable temperature to dissolve coarse eutectic intermetallic phases formed during casting process. Selecting the precise temperature and time of aging is an important task because these parameters significantly affect the final properties. In addition to their influence on mechanical properties and creep behavior, alloying elements should provide good castability (increased fluidity, low susceptibility to hot cracking, reduced porosity and greater casting integrity), further in combination with improved corrosion resistance and affordable cost. The development of new alloys usually requires to take into consideration both the desired performance and the affordable cost.
The gravity casting magnesium alloy ML11, developed in the former USSR, has been used for many years for applications at temperatures up to 200° C. This alloy contains 0.2-0.7 wt % Zn, 0.4-1.0 wt % Zr, 2.5-4.0 wt % RE, Ce-based mishmetal (typically containing 50 wt % Ce, 25 wt % La, 20 wt % Nd, 5 wt % Pr) with maximal impurity levels of (in wt %): Fe-0.01, Ni-0.005, Cu-0.03, Si-0.03, and Al-0.02. ML11 has relatively good creep resistance but exhibits very low ductility and impact strength as well as only moderate corrosion resistance.
U.S. Pat. No. 6,193,817 discloses magnesium-based alloy containing 0.1-2.0 wt % Zn, 2.1-5.0 wt % RE other than Y, up to 0.4 wt % of a combination of at least two elements chosen from the group consisting of Zr, Hf and Ti, and optionally up to 0.5 wt % Mn and up to 0.5 wt % Ca. This alloy has properties and disadvantages similar to those of ML11, with rather improved corrosion behavior.
U.S. Pat. No. 7,048,812 describes magnesium-based casting alloys containing 0.4-0.7 wt % Zn, 0.3-1.0 wt % Zr, 0.8-1.2 wt % RE (Ce based mishmetal), 1.4-1.9 wt % Nd. In fact this alloy is very similar to ML11 and the alloys of U.S. Pat. No. 6,193,817.
All these materials exhibit adequate creep behavior but have very low ductility, and energy-absorption properties. In addition, the above alloys are prone to hot tearing in the case of permanent mold casting technology.
U.S. Pat. No. 4,116,731 describes heat treated and aged magnesium based alloy containing 0.8-6.0 wt % Y, 0.5-4.0 wt % Nd, 0.1-2.2 wt % Zn, 0.3-1.1 wt % Zr, up to 0.05% Cu, and up to 0.2% Mn. Due to relatively wide concentration ranges claimed by the above patent, the alloys exhibit very diverse properties. However, all of them are prone to hot tearing under permanent mold casting, and exhibit poor corrosion behavior, low ductility and fracture toughness.
EP 1,329,530 discloses magnesium-based casting alloys containing 0.2-0.8 wt % Zn, 0.2-0.8 wt % Zr, 2.7-3.3 wt % Nd, 0.0-2.6 wt % Y, and 0.03-0.25% Ca. The alloys exhibit high strength and high creep resistance, but their ductility, and energy-absorption properties are not sufficient for engine cradle applications; furthermore, the alloys are relatively expensive and require high mold temperatures in order to avoid hot tearing formation under casting, especially when casting items having complicated geometries.
It is therefore an object of this invention to provide magnesium alloys suitable for permanent mold casting application, and to enable crack-free, not expensive, casting at mold temperatures as low as 300-320° C.
It is an object of this invention to provide magnesium-based alloys having high ductility and fracture toughness, as well as capability to operate at 200° C. for a long time.
It is another object of the present invention to provide alloys, which exhibit excellent combination of ductility, impact strength and fracture toughness, creep resistance, and corrosion resistance.
It is still a further object of this invention to provide alloys which exhibit the aforesaid behavior and properties, and have a relatively low cost.
Other objects and advantages of the present invention will appear as the description proceeds.